The inventor herein has previously designed and developed automatic controls for pellet mills which automatically control the process of converting dry material and various kinds of moisture including steam, water and molasses or the like into pellets of homogeneous composition as is desired in the feed and other industries. These automatic pellet mill controllers are disclosed and claimed in his prior patents including U.S. Pat. Nos. 3,932,736; 4,340,937; 4,463,430; and 4,700,310, the disclosures of which are incorporated herein by reference. Of particular significance with regard to the present invention disclosed and claimed herein is the '310 patent which discloses the use of a vapor steam generator which can convert water to steam on a continuous flow basis, producing superheated steam at a constant pressure of between 4-6 psi and an adjustable temperature between 194.degree. F.-600.degree. F. The operation of the vapor steam generator is more fully and completely disclosed in Pat. Nos. 4,211,071; 4,288,978; 4,337,619; 4,418,651; and 4,441,460, while its use in connection with a pellet mill is disclosed more particularly in the '310 patent mentioned above. Although reference should be made to these prior patents, the operation of the vapor steam generator, as understood by the inventor, generally has been to burn an air/gas mixture in a burner which is surrounded by water, the temperature of the exhaust steam being sensed downstream from the burner for use in modulating the flow of water through the combustion chamber to achieve a specific temperature. In other words, after an initial start-up of relatively short duration, the burner is operated at full capacity and the flow of water through the burner chamber is used to control the output temperature of the steam.
While in some processes it may be desirable for the output of a vapor steam generator to have a varying moisture content, in a pellet mill application it is highly desirable to control the moisture content of the steam more directly as one of the ultimate goals of a pellet mill is to produce pellets having a pre-determined moisture content. Such a pre-determined moisture content in a finished pellet may be more readily attained by controlling the rate of input of the dry material as well as the specific amount of moisture as the pellet mill operates. Of course, the pelleting process is a continuous process and not a batch process in that dry material and moisture are constantly being fed into the pellet mill. As explained in the inventor's prior patents, the pelleting process is not a simple one and there has been much effort expended to find ways to optimize the throughput capacity of the pellet mill while ensuring the production of satisfactory pellets having a desired moisture content. The vapor steam generator mode of controlling temperature by varying the water flow is not as desirable as it could be in that with its present control mode, there is a tendency for the vapor steam generator to produce steam having a varying moisture content.
To solve these and other problems in the prior art, and to more readily adapt the vapor steam generator for use in a pelleting process, the inventor herein has succeeded in developing a control mode for the vapor steam generator which permits it to generate a continuous flow of steam at a preselected moisture content, and yet provide temperature control of the steam produced by the vapor generator as is desired to control the temperature in the pellet mill. In simple terms, this mode of control comprises modulating the flow of the air/gas mixture to the burner while maintaining a relatively constant water flow through the combustion chamber. As in the previous control mode, the temperature of the exhaust steam output may be sensed shortly downstream from the combustion chamber. However, instead of using the water flow as the control parameter, the flow of air/gas to the burner may be used to increase or decrease the amount of heat generated and thereby change the temperature of the steam generated without changing the rate of flow of moisture. In its simplest form, the moisture may simply be ratioed as a percentage of the rate of flow of dry material which, after compensation for various losses due to boil off, etc. should result in finished pellets having the desired pre-determined moisture content.
Having satisfied the moisture content to be achieved in the final, finished pellets, the temperature range at which the pellet mill may operate could variably be chosen to optimize other parameters in the mill, such as pellet durability. For example, as the temperature in the pellet mill goes up, the friction in the die goes up which results in greater compaction of material in the pelleting process which results in a more compacted pellet having improved durability. Furthermore, higher temperatures may also increase the "cooking" effect which is desirably achieved in certain formulations to kill germs such as salmonella and other types of microorganisms. It has also been reported that higher temperatures add to the digestability of the finished pellet as the heat serves to break down the complex carbohydrates and other compounds contained in the feed. Still another consideration which is temperature dependent is the wear on the die. Absent these other considerations, it may be more desirable in some formulations to operate at lower temperatures which reduces die friction and helps to extend the operating life of the die.
In summary, it is believed that the present invention more completely adapts the vapor steam generator for use in the pelleting process by fixing the moisture rate while optimizing the steam temperature to suit the more important considerations inherent in the pelleting process and pellet mill. While the principal advantages and features of the present invention have been briefly described, a greater understanding thereof may be gained by referring to the drawings and description of the preferred embodiment which follow.